1. Field of the Invention
This invention relates to a plate making film and a plate making ink for the production of a printing plate used for printing such as offset printing, flexography or screen printing, and a plate making method and a plate making system using the plate making film and/or the plate making ink.
2. Description of the Related Art
Offset printing using a presensitized plate, flexography using a resinous letterpress, or screen printing using a photosensitive resin has employed a lith film containing a silver salt as a photosensitive material. Any of these printing methods rely on the following principle: A silver salt-containing film is set on the surface of a photosensitive layer of a plate material. Then, the photosensitive layer is irradiated with visible light or ultraviolet radiation through the film to solubilize or insolubilize the light- or ultraviolet-irradiated photosensitive portion by a photochemical reaction or the like. Then, the photosensitive layer is subjected to development to remove the soluble portion and form image areas corresponding to an image on the plate material.
The use of the lith film requires waste liquor disposal for the developing solution. Furthermore, equipment(s) for a development-printing-fixing system, and the lith film itself are very expensive. Thus, there has been a demand for development of an alternative to the lith film.
In recent years, electronic technologies have advanced remarkably. High performance models of computers have become available, and have been utilized in various fields. Output imaging machines for computers and recorders have also markedly advanced in technological improvement. Their spread to. the CAD field utilizing their high definition printout performance is promising, and may take the place of the lith film in the field of print making.
Image-forming or drawing machines are classified by their image forming principle into those based on several methods, such as the thermal printing method, the electrophotographic method, and the ink jet method. Alternatives to the lith film include, for example, a transparent film comprising a polyester film (e.g., PET) as a substrate, whose surface has been subjected to special treatment exclusive for each of the image forming methods. Any of these alternative films is used for plate making, and is thus required to have high dimensional stability of the base film as well as high resolution. This is a major factor for selection of the drawing method.
According to the thermal printing method and the electrophotographic method, heat of a head is continuously applied to the film, when an image is formed on the film. Thus, particularly when performing solid printing, the film stretches or deforms, thereby failing to obtain such accuracy as required of uses for plate making.
The ink jet method poses no such problems. However, this method involves a problem resulting from the properties of ink used, i.e., the problem that a hydrophilic membrane is indispensable for a surface to which ink is applied to draw an image, while a plastic film used as the transparent film is usually hydrophobic. A film having a hydrophilic membrane is disclosed in Japanese Patent Application Laying-open No. 6-47924 (1994), but this film is still problematical in terms of resolution. Moreover, the film used with the ink jet method is required to comprise a recording sheet excellent in ink wettability and absorbency, and free from stain due to escape of ink, bleeding of printed areas under high humidity, and stickiness of the printed surface.
To avoid the burden of using a plate making film for performing plate making, a proposal has been made for a direct plate making method which directly forms an image on a plate material by an image drawing device.
Among direct plate making systems known thus far is a laser plate making system. With this system, the surface of a plate material, such as a presensitized plate or an electrophotographic plate, is scanned with a beam of a laser which is turned on or off based on digital image information. A chemical or physical change of the portion irradiated with the laser beam forms image areas, corresponding to the image information, on the plate material. Such a laser plate making system, however, requires a laser device for generating a laser beam, and a complicated combination of an optical system, a drive system, and a scanning system for modulating a single laser beam, which is produced by the laser device, on the basis of image information, and guiding this beam to the surface of the plate material. Thus, a large-scale, expensive apparatus or system is used. Furthermore, the laser plate making system can produce various plates, but poses the problem that when a plate material including a silver salt as a photosensitive substance is used, the cost of the plate material is high, although high sensitivity and high resolution are obtained. When the laser plate making system is applied to an electrophotographic system which forms a toner image on a photosensitive member, the process involved is complicated, and the resolution obtained is not sufficient. With the system for laser plate making by use of a photopolymerizable printing plate as a plate material, the resolution and cost-effectiveness are high. However, the sensitivity is not sufficient, and a high power laser is required, thus making the system uneconomical.
For other direct plate making systems without use of laser, proposals have been made for methods employing a thermal drawing device such as a thermal head, and an ink drawing device. With the heat-sensitive plate making system, however, heat transmitted from the thermal head to a heat-sensitive recording surface is conveyed to a surrounding portion out of contact with the thermal head, thereby changing this portion. Thus, the boundary between image areas and non-image areas is not sharp, imposing limits on the resolution. The ink jet plate making system is free from this problem with the heat-sensitive plate making system, but involves increased complicacy, such as the need to select the most suitable ink depending on the type of the photosensitive resin on the plate material.
With textile printing or the like, a plate is usually used for a pattern portion, but a method of direct drawing onto a cloth by the ink jet method or the like has also been developed. In terms of productivity, however, the direct drawing method is inferior to a method which carries out textile printing after production of a plate. For mass production, the plate making method is still better.
As stated earlier, however, the conventional plate making method is high in cost, insufficient in the resolution and dimensional stability of the plate produced, complicated in procedure, or poor in mass producibility.
To make a plate with high resolution, ultraviolet rays are used to expose a photosensitive member. A portion of a film, where an image is drawn and ink is present, is required to have ultraviolet shielding properties as indispensable properties. Whereas a film portion, where no ink is present, must have ultraviolet transparency as an indispensable characteristic. The higher the contrast between these different properties, the better outcome will be obtained. Thus, it is necessary to combine an ink and a film which have such properties.
Ink jet recording method produces and flies ink droplets by various methods of ink ejection, such as electrostatic suction, a method of causing mechanical vibration or displacement to ink by the use of a piezoelectric element, and a method of heating ink to generate bubbles and push the ink under their pressure. By depositing the ink droplets onto a material to be recorded, ink jet recording makes records.
Ink used for ink jet recording is one consisting mainly of water because of safety and recording characteristics. Extensive studies on the ink have been conducted to prevent clogging of the nozzles and ensure the stability of ejection.
The material to be recorded, which is used for the ink jet recording method, is a transparent film which has been treated to acquire hydrophilicity. Water-soluble ink is ejected onto the so treated film to form an image pattern, thereby, for example, producing an OHP film. With the performance improvement and widespread use of ink jet recorders, more advanced properties are being demanded of materials to be recorded and of ink, and efforts to obtain a high resolution, high grade recorded image are continued.
It is an object of the present invention to provide a plate making film which is free from the drawbacks of the above-described earlier technologies, inexpensive, excellent in resolution, satisfactory in accuracy and dimensional stability, and good in mass producibility; and a plate making method and a plate making system which use the plate making film.
It is another object of the invention to provide a plate making ink giving a plate making film which is low in facility cost, convenient to use, free from excessive bleeding, and high in resolution and grade. The ink comprises a water-soluble dye, and forms an image pattern when ejected onto a transparent film as a material to be recorded in an ink jet printer, the transparent film having been treated to acquire hydrophilic properties.
The plate making film according to the present invention is characterized by having a transparent substrate, and a layer provided on the substrate, the layer consisting essentially of a polymeric compound composed of a repeating unit represented by the following general formula (1):
"Brketopenst"AXAR1"Brketclosest"xe2x80x83xe2x80x83(1)
wherein
A comprises
"Parenopenst"CH2CH2O"Parenclosest"
xe2x80x83and 
xe2x80x83the mode of addition of A being 
x, y and z each denote an integer of 1 or more,
the weight ratio calculated from x, y and z, i.e., 44(x+z)/(the molecular weight of an xcex1-olefin oxide having 3 or more carbon atoms)y, ranges from 80/20 to 94/6,
X is a residue of an organic compound having 2 active hydrogen groups,
Y is a hydrocarbon group having 1 or more carbon atoms, and
R1 is a dicarboxylic acid compound residue or a diisocyanate compound residue.
If the above-mentioned weight ratio, i.e., 44(x+z)/(the molecular weight of an xcex1-olefin oxide having 3 or more carbon atoms)y, is less than 80/20, the hydrophilicity declines, resulting in poor ink absorptivity and poor printability. If the weight ratio exceeds 94/6, on the other hand, the resulting film will be poor in terms of ink bleeding and water resistance. Y has the function of imparting hydrophobicity to the polymeric compound of the general formula (1). Preferred examples of Y are alkyl groups such as an ethyl group. By setting the proportions of x, y and z at those which satisfy the range defined above, the resulting film can become insoluble in water without losing hydrophilicity. Thus, the film of the present invention, unlike the transparent film in Japanese Patent Application Laying-open No. 6-47924 (1994), can prevent ink from spreading gradually after printing.
The plate making film desirably has light transmittance at 400 nm of 10 to 40%.
The plate making method according to the present invention is characterized by including the step of:
drawing an image on the surface of the layer consisting essentially of the polymeric compound of the aforementioned plate making film;
bringing the drawn plate making film into intimate contact with a photosensitive member;
exposing the photosensitive member through the plate making film; and
stripping the plate making film from the photosensitive member, and developing the photosensitive member.
In this method, the drawing step is desirably performed by ink jet recording.
The plate making system according to the present invention is characterized by including ink jet drawing means for drawing a predetermined image on a surface of a transparent plastic film with an ultraviolet absorbing ink, the transparent plastic film being hydrophilic and ink absorptive at least on the surface; exposure means for exposing a photosensitive member to ultraviolet radiation by use of the drawn transparent plastic film as a photomask; and developing means for developing the exposed photosensitive member to prepare a printing plate.
Preferably, the transparent plastic film is the aforementioned plate making film having a transparent substrate, and a layer provided on the substrate, the layer essentially consisting of a polymeric compound composed of a repeating unit represented by the general formula (1).
The drawing means, exposure means, and developing means in the plate making system may be drawing means, exposure means, and developing means for production of a large image plane.
Both of a presensitized plate for offset printing, and a photosensitive resin plate for screen printing and flexography form an image, when intimately contacted with a plate making film, and irradiated with ultraviolet radiation from a high pressure mercury lamp, a metal halide lamp, a chemical lamp or the like. Thus, an image drawn on the plate making film, in other words, an ink for drawing the image, needs to cut 99.99% or more of ultraviolet radiation. In addition, the ink, as stated earlier, must be excellent in wetting properties for and absorbability to the plate making film, and free from stain due to escape of ink, excessive bleeding of a printed image plane under high humidity, and stickiness of the printed surface.
That is, the plate making ink according to the present invention is a plate making ink for use on a plate making film having a layer provided on a transparent substrate, the layer consisting essentially of a polymeric compound composed of a repeating unit represented by the aforesaid general formula (1), the ink being characterized by containing a water-soluble azo dye selected from mono- or disazo dyes, the azo dye being a yellow dye highly absorptive for light with a wavelength of 350 to 450 nm.
Preferably, the water-soluble azo dye contains an acetoacetic anilide bond. Further preferably, the aforementioned plate making film is a film for a presensitized plate for offset printing, or a film for making a plate for screen printing or flexography, and the aforesaid plate making ink is ink for direct drawing onto the plate making film by means of an ink jet printer.
The plate making ink is preferably such that D/T, the ratio of the diameter D and the depth T of a dot in an image printed on the plate making film described above by means of an ink jet printer using the plate making ink, i.e., D/T, is 1.1 less than D/T less than 8.
The yellow dye may be contained in an amount of 1 to 10% by weight based on the total weight of the plate making ink.
Predetermined additives may be contained in an amount of 5 to 40% by weight based on the total weight of the plate making ink.
Ethanol may be contained in an amount of 3 to 12% by weight based on the total weight of the plate making ink.
The above and other objects, as well as the effects, features and advantages of the present invention will become clearer by reference to the Examples offered below and the accompanying drawings.